The electrolytic capacitor is a condenser using as a dielectric an insulating oxide layer formed by subjecting a surface of a metal foil such as aluminum or tantalum to an electrolytic oxidation treatment to form the insulating oxide layer, and is large in the capacitance because the accessible electrode area and the dielectric constant of the insulating film are large.
The usual aluminum electrolytic capacitor element using aluminum is a structure wherein a band-shaped positive electrode foil having an insulating oxide layer formed through anodic oxidation and a band-shaped negative electrode foil having no insulating oxide layer are cylindrically wound through a separator, or a structure in which the separator is sandwiched between the positive electrode and the negative electrode and they are alternately laminated or folded. The electrolytic capacitor element is impregnated with an electrolyte and placed in an armored casing and further an opening portion of the armored casing is hermetically sealed with a sealing material to form an electrolytic capacitor. As the electrolyte is usually used a solution obtained by dissolving boric acid or carboxylic acid, or an ammonium salt thereof in an organic solvent such as ethylene glycol, N,N-dimethyl formamide, γ-butyrolactone or the like.
In the above electrolytic capacitor, as the internal pressure of the element rises due to heat, overpotential or the like, there may be caused a case that the electrolyte leaks out from the sealed portion, or an explosion-proof valve is actuated to blow out the electrolyte. In this case, there is a possibility that the leaked electrolyte is ignited by a spark due to the short-circuiting of the electrolytic capacitor itself or a spark from the other electronic part to damage the device or cause the fire. Recently, electronics devices utilizing the electrolytic capacitor become widely used even in general households. For this end, it is demanded to develop electrolytes having a high safety even if the electrolyte is leaked to outside due to the troubles of the electrolytic capacitor. However, the aforementioned solution of boric acid or carboxylic acid or the ammonium salt thereof in the organic solvent has a problem that the safety is insufficient.
On the other hand, as a flame retarder for plastics are known phosphorus compounds, halogen compounds, antimony oxide and the like, but when the flame retardance is given to the electrolyte, it is important that the basic performances of the electrolyte (service temperature range, conductivity, spark voltage, compatibility to electrode and so on) are not lowered. For example, it is generally considered that solid materials lower the conductivity and halides corrode the electrode and hence they can not be included in the electrolyte. With this in mind, there is proposed a technique that non-halogen phosphoric ester is added to the electrolyte to improve the flame retardance of the electrolytic capacitor (see Japanese Patent No. 2611262). However, the flame retardance is insufficient even in this electrolytic capacitor in accordance with use environments, so that it is demanded to develop electrolytic capacitors further improving the flame retardance.